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Fibers describe essentially the same concept as coroutines. The distinction, if there is any, is that coroutines are a language-level construct, a form of control flow, while fibers are a systems-level construct, viewed as threads that happen not to run in parallel. Priority is contentious; fibers may be viewed as an implementation of coroutines,[1] or as a substrate on which to implement coroutines.[2]

Because fibers multitask cooperatively, thread safety is less of an issue than with preemptively scheduled threads, and synchronization constructs including spinlocks and atomic operations are unnecessary when writing fibered code, as they are implicitly synchronized. However, many libraries yield a fiber implicitly as a method of conducting non-blocking I/O; as such, some caution and documentation reading is advised. A disadvantage is that fibers cannot utilize multiprocessor machines without also using preemptive threads; however, an M:N threading model with no more preemptive threads than CPU cores can be more efficient than either pure fibers or pure preemptive threading.

In modern server programs fibers are used to soft block themselves to allow their single-threaded parent programs to continue working. In this design, fibers are used mostly for i/o access which does not need cpu processing. This allows the main program to continue with what it is doing. Fibers yield control to the single-threaded main program, and when the i/o operation is completed fibers continue where they left off.

On Microsoft Windows, fibers are created using the ConvertThreadToFiber and CreateFiber calls; a fiber that is currently suspended may be resumed in any thread. Fiber-local storage, analogous to thread-local storage, may be used to create unique copies of variables.[3]

Symbian OS used a similar concept to fibers in its Active Scheduler. An active object contained one fiber to be executed by the Active Scheduler when one of several outstanding asynchronous calls completed. Several Active objects could be waiting to execute (based on priority) and each one had to restrict its own execution time.